Séminaire

About 80 tropical cyclones form annually; a rate that has been remarkably steady over the period of reliable historical record and that continues to mystify climate scientists. Roughly two thirds of these storms form in the Northern Hemisphere, from about June to November, while the remaining one third form in the Southern Hemisphere during the months of November through May. Our understanding of the global and regional spatial patterns, the year-to-year variability, and temporal trends of these storms has improved considerably since the advent of meteorological satellites in the 1960s, owing to advances in both remote sensing technology and operational analysis procedures. That said, the well-recognized spatial patterns of tropical cyclone formation and tracks were laid out in a series of seminal papers in the late 1960s and 1970s and remain an accurate sketch even to this day. Concerning the year-to-year variability of tropical cyclone frequency, the El Niño Southern Oscillation (ENSO) has by far the most dominant influence across multiple ocean basins; so much so that it is typically used as the main predictor for statistical forecasts of seasonal tropical cyclone activity.
In the Australian region, seasonal tropical cyclone counts (TCC) maintained a robust statistical relationship with ENSO, with skillful forecasts of above (below) average TCC during La Niña (El Niño) years from 1969 until about 1998, weakening thereafter.  An additional climate driver is identified here that mitigates the loss of predictive skill for Australian TCC after about 1998. It is found that the seasonal Australian TCC is strongly modulated by a southwest-to-northeast-orientated dipole in Indian Ocean sea surface temperature anomalies (SSTA), referred to here as the Transverse Indian Ocean Dipole (TIOD). TIOD emerges as the leading mode of detrended Indian Ocean SSTA in the Southern Hemisphere during late winter and spring. Active (inactive) TC seasons are linked to positive (negative) TIOD phases, most notably during August-October immediately preceding the TC season. We show that the combination of the TIOD and Niño 4 provide a skill increase (up to 33%) over climatology, demonstrating reliably accurate seasonal predictions of Australian region TCC.

marie-pierre.lefebvre@lmd.jussieu.fr